A character is too dense to have any buoyancy, so they deal with bodies of water by walking across the bottom rather than swimming. This is a common strategy with characters who can breathe underwater, or creatures like zombies, golems, or robots who don't need to breathe at all. Of course, immunity to drowning isn't strictly necessary to walk underwater, but it helps. If this is done by Fish People who should be capable of swimming, for no reason other than ignorance of the third dimension, see Water Is Air. See Walk On Water for an even more impressive version.